Some may be disposed to consider it as an imperfection in this picture, that all the objects appear in an inverted position; as they must necessarily do, according to what we formerly stated respecting the properties of convex lenses, (p. 72). There are, however, different modes of viewing the picture as if it were erect. For, if we stand before the picture, and hold a common mirror against our breast at an acute angle with the picture, and look down upon it, we shall see all the images of the objects as if restored to their erect position; and by the reflection of the mirror, the picture will receive such a lustre as will make it still more delightful. Or, if a large concave mirror were placed before the picture at such a distance, that its image may appear before the mirror, it will then appear erect and pendulous in the air in the front of the mirror. Or, if the image be received on a frame of paper, we may stand behind the frame, with our face towards the window, and look down upon the objects, when they will appear as if erect.
The experiment of the Camera Obscura may serve to explain and illustrate the nature of a common refracting telescope. Let us suppose, that the lens in the window-shutter represents the object-glass of a refracting telescope. This glass forms an image in its focus, which is in every respect an exact picture or representation of the objects before it; and consequently the same idea is formed in the mind, of the nature, form, magnitude, and colour of the object—whether the eye at the centre of the glass views the object itself, or the image formed in its focus. For, as formerly stated, the object and its image are both seen under the same angles by the eye placed at the centre of the lens. Without such an image as is formed in the camera obscura—depicted either in the tube of a telescope or in the eye itself—no telescope could possibly be formed. If we now suppose that, behind the image formed in the dark chamber, we apply a convex lens of a short focal distance to view that image, then the image will be seen distinctly, in the same manner as we view common objects, such as a leaf or a flower, with a magnifying glass; consequently, the object itself will be seen distinct and magnified. And, as the same image is nearer to one lens than the other, it will subtend a larger angle at the nearest lens, and of course, will appear larger than through the other, and consequently the object will be seen magnified in proportion. For example, let us suppose the lens in the camera obscura, or the object lens of a telescope, to be five feet, or sixty inches focal distance, at this distance from the glass, an image of the distant objects opposite to it will be formed. If now, we place a small lens two inches focal distance beyond this point, or five feet two inches from the object-glass, the objects, when viewed through the small lens, will appear considerably magnified, and apparently much nearer than to the naked eye. The degree of magnifying power is in proportion to the focal distances of the two glasses; that is, in the present case, in the proportion of two inches, the focus of the small lens, to sixty inches, the focus of the object lens. Divide sixty by two, the quotient is thirty, which gives the magnifying power of such a telescope, that is, it represents objects thirty times nearer, or under an angle thirty times larger than to the naked eye. If the eye-glass, instead of being two inches, were only one and a half inch focus, the magnifying power would be in the proportion of one and a half to sixty, or forty times. If the eye-glass were three inches focus, the magnifying power would be twenty times; and so on, with regard to other proportions. In all cases, where a telescope is composed of only two convex lenses, the magnifying power is determined, by dividing the focal distance of the object-glass, by the focal distance of the eye-glass, and the quotient expresses the number of times the object is magnified, in length and breadth. This and various other particulars, will be more fully illustrated in the sequel.
In performing experiments with the camera obscura in a darkened chamber, it is requisite that the following particulars be attended to:—1. That the lens be well figured, and free from any veins or blemishes that might distort the picture. 2. That it be placed directly against the object whose image we wish to see distinctly delineated. 3. The lens should be of a proper size both as to its breadth and focal distance. It should not be less than three or four feet focal distance, otherwise the picture will be too small, and the parts of objects too minute to be distinctly perceived; nor should it exceed fifteen or eighteen feet, as in this case the picture will be faint, and of course not so pleasing. The best medium as to focal distance, is from five to eight or ten feet. The aperture, too, or breadth of the glass, should not be too small, otherwise the image will be obscure, and the minute parts of it invisible for want of a sufficient quantity of light. A lens of six feet focal distance, for example, will require an aperture of at least two inches. Lenses of a shorter focal distance require less apertures, and those of a longer focal distance larger. But if the aperture be too large, the image will be confused, and indistinct, by the admission of too much light. 4. We should never attempt to exhibit the images of objects, unless when the sun is shining and strongly illuminating the objects, except in the case of very near objects placed in a good light. As one of the greatest beauties, in the phenomena of the dark chamber, consists in the exquisite appearance and contrast of light and shadows, nothing of this kind can be perceived but from objects directly illuminated by the sun. 5. A south window should never be used in the forenoon, as the sun cannot then enlighten the north side of an object; and besides, his rays would be apt to shine upon the lens, which would make the picture appear with a confused lustre. An east window is best in the afternoon, and a western in the morning; but a north window is in most cases to be preferred, especially in the forenoon, when the sun is shining with his greatest strength and splendour. In general, that window ought to be used which looks to the quarter opposite to that in which the sun is shining.
The picture should be received upon a very white surface, as the finest and whitest paper, or a painted cloth, bordered with black; as white bodies reflect most copiously the incident rays, while black surfaces absorb them. If the screen could be bent into the concave segment of a sphere, of which the focal distance of the double convex lens which is used, is the radius, the parts of the picture adjacent to the extremities would appear most distinct. Sir D. Brewster informs us that, having tried a number of white substances of different degrees of smoothness, and several metallic surfaces, on which to receive the image, he happened to receive the picture on the silvered back of a looking-glass, and was surprised at the brilliancy and distinctness with which external objects were represented. To remove the spherical protuberances of the tin foil, he ground the surface very carefully with a bed of hones which he had used for working the plane specula of Newtonian telescopes. By this operation, which may be performed without injuring the other side of the mirror, he obtained a surface finely adapted for the reception of images. The minute parts of the landscape were formed with so much precision, and the brilliancy of colouring was so uncommonly fine, as to equal, if not exceed the images that are formed in the air by means of concave specula.
The following additional circumstances may be stated respecting the phenomena exhibited in the dark chamber. A more critical idea may be formed of any movement in the picture here presented than from observing the motion of the object itself. For instance, a man walking in a picture appears to have an undulating motion, or to rise up and down every step he takes, and the hands seem to move almost exactly like a pendulum; whereas scarcely any thing of this kind is observed in the man himself, as viewed by the naked eye. Again, if an object be placed just twice the focal distance from the lens without the room, the image will be formed at the same distance from the lens within the room, and consequently will be equal in magnitude to the object itself. The recognition of this principle may be of use to those concerned in drawing, and who may wish, at any time, to form a picture of the exact size of the object. If the object be placed further from the lens than twice its focal length, the image will be less than the object. If it be placed nearer, the image will be greater than the life. In regard to immoveable objects, such as houses, gardens, trees, &c., we may form the images of so many different sizes, by means of different lenses, the shorter focus making the lesser picture, and the longer focal distance the largest.
The experiments with the camera obscura, may likewise serve to illustrate the nature of vision, and the functions of the human eye. The frame or socket of the scioptric ball may represent the orbit of the natural eye. The ball, which turns every way, resembles the globe of the eye, moveable in its orbit. The hole in the ball may represent the pupil of the eye; the convex lens corresponds to the crystalline humour, which is shaped like a lens, and contributes to form the images of objects on the inner part of the eye. The dark chamber itself, is somewhat similar to the internal part of the eye, which is lined all around, and under the retina, with a membrane, over which is spread a mucous of a very black colour. The white wall or frame of white paper to receive the picture of objects, is a fair representation of the retina of the eye, on which all the images of external objects are depicted. Such are some of the general points of resemblance between the apparatus connected with the dark chamber, and the organ of vision; but the human eye is an organ of such exquisite construction, and composed of such a number and variety of delicate parts, that it cannot be adequately represented by any artificial contrivance.
The darkened chamber is frequently exhibited in a manner somewhat different from what we have above described, as in the following scheme, (fig. 38) which is termed the revolving camera obscura. In this construction, KH represents a plane mirror or metallic reflector, placed at half a right angle to the convex lens HI, by which, rays proceeding from objects situated in the direction O are reflected to the lens, which forms an image of the objects on a round white table at T, around which several spectators may stand, and view the picture, as delineated on a horizontal plane. The reflector, along with its case, is capable of being turned round, by means of a simple apparatus connected with it, so as to take in, in succession, all the objects which compose the surrounding scene. But as the image here is received on a flat surface, the rays fm, en, will have to diverge farther than the central rays dc; and hence the representation of the object, near the sides, will be somewhat distorted; to remedy which, the image should be received on a concave surface, as ab or PS. This is the general plan of those Camera Obscuras, fitted up in large wooden tents, which are frequently exhibited in our large cities, and removed occasionally from one town to another. Were an instrument of this kind fitted up on a small scale, a hole might be made in one of the sides, as at E, where the eye could be applied to view the picture. The focal distances of the lenses used in large instruments of this kind, are generally from eight to twelve feet, in which case they produce a telescopic effect upon distant objects, so as to make them appear nearer than when viewed with the naked eye.
figure 38.
